Synchronous rectification is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches such as transistors, usually power metal oxide semiconductor field effect transistors (MOSFETs). Replacing a diode with an actively controlled switching element such as a MOSFET is at the core of active rectification. The MOSFETs have a constant very low resistance when conducting, known as on-resistance RDS (on). They can be made with an on-resistance as low as about 10 mΩ or even lower. The voltage drop across the MOSFET is then much lower than a diode, which results in a reduction in power loss and a gain in efficiency. At high currents, however, the drop can exceed that of a diode. To accommodate the increase in drop at high currents, several transistors can be placed in parallel, thereby reducing the current through each transistor, or by using a transistor device with more active area.
The control circuitry for active rectification circuits sense the voltage of the input AC and control the transistors to allow current to flow in the correct direction for AC to DC conversion. The timing is important, as timing errors can cause a short circuit as well as reduce the overall efficiency of the power converter.